The invention relates to a method of manufacturing a device in at least one layer on a substrate, comprising the steps of:
imaging, by means of projection radiation having a wavelength xcex and a projection system having a numerical aperture NA, a specific phaseshifting mask pattern, comprising pattern features corresponding to device features to be configured in said layer, on a radiation-sensitive layer provided on said layer, and
removing material from, or adding material to, areas of said layer which are delineated by the mask pattern image, the smallest device features having a width which is smaller than xcex/NA.
The invention also relates to a lithographic phaseshifting mask for use in this method.
The method is used, inter alia, in the manufacture of integrated electronic circuits, or IC, devices. An IC mask pattern, present in a mask, is imaged each time on a different IC area of a substrate. This substrate, which is coated with a radiation-sensitive layer, provides space for a large number of IC areas. The lithographic method may also be used in the manufacture of other devices like, for example, integrated or planar optical systems, charge-coupled detectors (CCDs), magnetic heads or liquid crystalline display panels.
Since it is desirable to accommodate an increasing number of electronic components in an IC device, increasingly smaller features, or line widths, of IC patterns must be imaged. Thus, increasingly stricter requirements are imposed on the lithographic projection apparatus used to carry out the lithographic method. The special requirements relate especially to the imaging quality and the resolving power of the projection system, which is usually a lens system in the current lithographic apparatus. The resolving power, or resolution, which is a measure of the smallest feature which can still be imaged satisfactorily, is proportional to xcex/NA, in which xcex is the wavelength of the imaging, or projection, beam and NA is the numerical aperture of the projection system. To increase the resolution, the numerical aperture may, in principle, be increased and/or the wavelength may be reduced. In practice, an increase of the numerical aperture, which is currently already fairly large, is not very well possible because this reduces the depth of focus of the projection lens system, which is proportional to xcex/NA2 and, moreover, it becomes too difficult to correct for the required image field.
The requirements to be imposed on the projection lens system may be alleviated, or the resolution may be increased, while maintaining these requirements, if a step-and-scanning lithographic apparatus is used instead of a stepping lithographic apparatus. In a stepping apparatus, a full-field illumination is used, i.e. the entire mask pattern is illuminated in one operation and imaged as a whole on an IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area is positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate are provided with an image of the mask pattern. In a step-and-scanning apparatus, only a rectangular or circular segment-shaped area of the mask pattern and hence also a corresponding sub-area of the substrate IC area is each time illuminated, and the mask pattern and the substrate are synchronously moved through the illumination beam, while taking the magnification of the projection lens system into account. A subsequent sub-area of the mask pattern is then each time imaged on a corresponding sub-area of the relevant IC area of the substrate. After the entire mask pattern has been imaged on an IC substrate area in this way, the substrate holder performs a stepping movement, i.e. the beginning of the next IC area is moved into the projection beam and the mask is set, for example, in its initial position whereafter said next IC area is scan-illuminated via the mask pattern.
If even smaller features are to be imaged satisfactorily with a stepping or a step-and-scanning lithographic apparatus, use can be made of a phase-shifting mask. The technique for improving resolution in photolithography by the use of phase-shifting masks was first proposed by Levenson et al in: xe2x80x9cImproving Resolution in Photolithography with a Phase-Shifting Maskxe2x80x9d, IEEE Transactions on Electron Devices, Vol. ED-29, No.12, December 1982, pp 25-32. The Levenson phase-shifting mask is a conventional transmission mask which is provided with phase-shifting elements. This transmission mask comprises a transparent, for example, quartz substrate covered by an opaque, for example, chrome layer with apertures to define the desired intensity pattern, i.e. the IC pattern to be printed in a layer of the IC device. When illuminating such a conventional mask with electromagnetic radiation, the electric field of this radiation has the same phase at every aperture. However, due to diffraction at the apertures and the limited resolution of the projection lens system, the electric field patterns at the substrate level are spread. A single small mask aperture thus provides a wider intensity distribution at substrate level. Constructive interference between waves diffracted by adjacent apertures enhances the electric field between the projections of the apertures at substrate level. As the intensity pattern is proportional to the square of the electric field, this pattern of two adjacent mask apertures is spread evenly to a fairly high degree and does not show two pronounced peaks at the positions of the apertures.
In a phase-shifting mask, one of the two adjacent apertures is covered with a transparent phase-shifting layer. This layer has a thickness d=xcex/2(nxe2x88x921), where n is the index of refraction and xcex is the wavelength of the radiation, such that the waves transmitted through the adjacent apertures are 180xc2x0 out of phase with one another. Destructive interference now occurs between the waves diffracted by the adjacent apertures, and the electric field, and thus the intensity, between the projections of the apertures at wafer level is minimised. Any projection lens system will project the images of such a phase-shifting mask with a better resolution and a higher contrast than a corresponding mask without phase shifters.
A similar improvement can be obtained by a xe2x80x9cchrome-lessxe2x80x9d phase shifting mask, as disclosed in EP-A 0.680.624. This mask does not comprise a pattern structure of chrome, or other opaque material, which defines the IC pattern, but this pattern is now defined by a pattern of phase transitions, for example, in the form of recesses in the quartz substrate. Such a phase transition, or pattern feature, is imaged by the projection lens system in the radiation-sensitive, or resist, layer on the wafer as a narrow line, due to the point-spread function of this lens system. The line width is typically below 100 nm and can be influenced by the numerical aperture of the projection lens system, the coherence value and the exposure dose of the lithographic apparatus. The coherence value, or "sgr" value, is the ratio of the cross-section of the projection beam in the plane of the pupil of the projection lens system and the aperture of this lens system. The "sgr" value thus indicates the degree in which the projection lens pupil is filled by the projection beam. This value is usually smaller than one. The exposure dose is the amount of projection, or exposure, radiation incident on a resist layer area during imaging of a mask feature on this area. Once the numerical aperture, the coherence value and the exposure dose are set for a lithographic apparatus, all pattern features projected in the resist layer, and later configured in the IC device layer, have the same width.
If, as is usually the case, IC features with different widths should be formed in a device layer, it is not one mask but a number of masks, corresponding to the number of different widths, that should be projected on this layer. Each mask should then be projected with different values for the parameters NA, "sgr" and exposure dose. This is a cumbersome and time-consuming process which would considerably increase the manufacturing time of an IC device or another device.
It is an object of the present invention to solve the above problem and to provide a manufacturing method by means of which pattern features having considerably different widths can be configured in a device layer in one illumination step. This manufacturing method is characterized in that use is made of a mask pattern comprising mask features which are constituted by the combination of a phase transition determining the position of the imaged mask feature in the device layer and the length of the imaged feature, and two sub-resolution assist features flanking the phase transition and having a specific mutual distance which substantially determines the width of the imaged mask feature in the device layer.
A sub-resolution assist feature is a feature having such a small width that it is not resolved by the projection lens system of the lithographic apparatus, i.e. it is not imaged as such by the projection lens system. However, due to their diffractive effects, the two assist features flanking a phase transition determine the width of the image of this phase transition. This width of the phase transition image is mainly determined by the distance between the assist features. For example, by varying said distance between 200 nm and 600 nm, the width of a device feature can be set accurately to any value from 270 nm down to 50 nm. These dimensions are dimensions in the plane of the layer to be configured, i.e. dimensions at substrate level. Such a notation is commonly used in lithography. For a lithographic projection apparatus having a magnification M=xc2xc, the feature dimensions at mask level are four times larger than the corresponding dimensions at substrate level.
The present invention is based on the recognition that the two functions of determining the position of a device feature and determining the width of this feature, which functions are performed in a conventional mask by one corresponding mask feature, can be separated and performed by three mask features. The position of the device feature in the device layer is determined by the position of the corresponding phase transition in the mask pattern, and the width of the device feature is determined by the two assist features. For printing device features with different widths, no special requirements have to be imposed on the lithographic apparatus. The assist features are not imaged, but improve the contrast of the phase transition image. As the assist features are not imaged, they can be used without any adaptation of the lithographic apparatus. By using the concept of the present invention, an existing lithographic apparatus can simultaneously print device features having different widths, ranging from very small to, for example, five times the minimum width.
The sub-resolution assist features can be characterized as scattering bars. It is to be noted that the use of scattering bars in the technique of optical lithography is known per se. For example, U.S. Pat. No. 5,242,770 relates to the problem that isolated features, i.e. features which have no other features in their neighbourhood, are imaged in the device layer as smaller device features than the images of densely packed mask features having the same width. To solve this problem, it is proposed to arrange an intensity gradient leveling bar, having a sub-resolution width and acting as a scattering bar, at each isolated edge of the isolated features. These leveling bars should adjust the edge intensity gradients of isolated edges in the mask pattern to the edge intensity gradients of densely packed edges. An intensity gradient leveling bar effects an increase of the width of the device feature of the order of 10% and thus performs a fine tuning of the device feature width. Moreover, all intensity gradient leveling bars are arranged at the same small distance from the isolated edges, which distance is of the order of 1.1 of the critical dimension of the device pattern. The mask used in the method described in U.S. Pat. No. 5,242,770 is not a phase-shifting mask, but an amplitude mask in the form of a clear field mask or a dark field mask.
PCT application WO 99/47981 proposes the use of sub-resolution features in an attenuated phase-shifting mask to solve another problem. The problem is that the image of an original mask feature, having a width which is larger than the minimal width, does not have a uniform intensity, but shows a certain intensity distribution. To solve this problem, the original mask feature, having a width of the order of 1 xcexcm, is de-composed into an array of phase-shifted xe2x80x9cimaging elementsxe2x80x9d which are separated by non-phase shifting and sub-resolution elements referred to as anti-scattering bars. Additional scattering bars may be arranged at the edges of the original mask feature to improve the depth of focus.
The method is preferably, further characterized in that the width of the imaged mask feature is tuned by adapting at least one of the following parameters:
the width of the assist features;
the transmission of the assist features, and
the phase shift introduced by the assist features.
Adaptation of these parameters allows a fine tuning, and thus an optimisation, of the width of the device features.
A first embodiment of the method is characterized in that use is made of a mask pattern wherein the assist features are constituted by opaque strips.
The term opaque strip should be broadly interpreted as a strip which prevents radiation incident thereon from following the same path as radiation incident outside the strip. For a transmission mask, an opaque strip is a non-transmission strip, for example, a reflective or absorbing strip. For a reflection mask, an opaque strip is a non-reflective strip.
In principle, an assist feature may also be constituted by a phase transition. In order to prevent such an assist feature from being imaged in the resist layer it should be considerably smaller than an opaque strip having the same performance. A mask having phase transition assist features is more difficult to manufacture than a mask having opaque strip assist features.
A second, embodiment of the method which, in view of the mask manufacture is even preferred, is characterized in that use is made of a mask pattern wherein the assist features are constituted by radiation attenuation strips.
This embodiment uses the same principle as the method of patterning with an attenuated phase-shifting mask, described in the above-mentioned PCT application WO 99/47981. In the attenuated phase-shifting mask, the device features are strips showing a smaller transmission than their surroundings, whereby the difference in transmission is of the order of 5%. The assist features used in the method of the present invention have a transmission which is, for example, of the order of 75% of that of their surroundings. An assist feature with such a transmission may have a width which is, for example, of the order of 166% of the width of an opaque assist features, so that a mask pattern with attenuated assist feature is easier to manufacture than a mask pattern with opaque assist features.
The invention can also be implemented by means of the so-called trim mask technology. The method of configuring IC patterns by means of a trim mask is described in the article: xe2x80x9cThe application of alternating phase-shifting mask to 140 nm gate patterning (2); Mask design and manufacturing tolerancesxe2x80x9d in SPIE, VOL 3334, 1998, page 2. The trim mask method uses two masks which are successively projected on the same area of a substrate layer. The first mask is a phase-shifting mask with a phase transition at the position of a device feature, for example, a transistor gate. This mask is used exclusively to define the transistor gate. The second mask, i.e. the trim mask, is a chrome mask which protects the narrow gates defined by the first mask, removes unwanted edges produced by the exposure with the first mask and defines the remaining interconnect pattern. In the first mask, a device feature is defined by a 180xc2x0 transition in a clear area. A chrome layer surrounds this area, which may be called a phase-shifting area. The width of this area determines the width of the image of the mask device feature. The width of the phase-shifting area can be chosen to be such that the variation of the feature image width, due to different kinds of projection lens aberrations, is minimised. However, this aberration-free imaging is possible only for one feature width, the optimum width.
In a third embodiment of the method of the invention, which is characterized in that use is made of a mask pattern wherein the phase transitions are arranged in phase-shifting areas which are embedded in opaque surroundings, and the assist features belonging to a specific phase transition are arranged within its phase-shifting area, the above-mentioned aberration-free imaging can be performed for different feature widths. The optimum feature width can now be varied over a wide range by changing the distance between the assist features.
The invention also relates to a phase-shifting mask for optically transferring a mask pattern, corresponding to a device layer structure, from said mask to a layer on a substrate of a device. This mask is characterized in that the mask pattern comprises mask features which are constituted by the combination of a phase transition determining the position of the imaged mask feature in the device layer and the length of the imaged mask feature, and two sub-resolution assist features flanking the phase transition and having a specific mutual distance, which distance substantially determines the width of the imaged mask feature in the device layer.
This mask may be further characterized in that the phase transitions are arranged in phase-shifting areas which are embedded in opaque surroundings, and the assist features belonging to a specific phase transition are arranged within its phase-shifting area.
Such a mask is suitable for use in the trim mask method.
A first embodiment of the mask is characterized in that the mask pattern is a transmission pattern.
A transmission mask pattern is usually applied in conventional lithography using UV and deep UV radiation.
A second embodiment of the mask is characterized in that the mask pattern is a reflective pattern.
The embodiments may be characterized in that the assist features are constituted by opaque strips.
The embodiments may be further characterized in that the material of the strips is chrome.
Chrome has proved to be a material which is very well suitable for delineating a mask pattern. It has been established that chrome is also very suitable for scattering bars in the new application according to the present invention.
Alternatively, the embodiments may be characterized in that the assist features are constituted by radiation attenuating strips.
The latter embodiments may be further characterized in that the material of the strips is a composition of molybdenum and silicon.
The transmission coefficient of the material MoSi renders this material very suitable for attenuating strips.